Just keep in mind that many LFS don't know what they're getting, either. There are dozens of LFS in my area that sell columbian rams (who knows where they're getting them) and even a couple selling canas and spixis. When I found them, I was in total shock. I asked the owner where he got them from and he said they were sold to him as "pond snails" from a wholesaler in Georgia. He had no idea what they were, just thought they were normal aquarium snails, nothing odd. I had to explain to him how to ID shipping-restricted species - but that was made even more difficult by the fact he had brigs in there too.
As far as I know the USDA has no right to take snails that you bought legally, even a shipping-restricted species, as long as the snail is legal to own in your state. By all means, cooperate with them and please be polite as they are only doing their jobs, but they shouldn't just seize snails if you purchased them legally.
And if you do own a shipping-restricted species, you may want to take care to show that you are being responsible and controlling the population. If they visited my house they would find dixie cups in my freezer full of unwanted clutches and egg sacs. They would find a tank full of assassins for taking care of any babies that hatch in the tank accidentally. They would see a closed system and covered tank full of legally purchased snails. Just be responsible, keep everything transparent and easy to inspect, and you shouldn't have any problems.